Electronic copying equipment with controlled heater

ABSTRACT

A temperature varying resistor is in one branch of a bridge circuit. The voltage across the diagonal of the bridge controls the heater cut in and cut off under normal operation. The voltage across the temperature varying resistor is applied to two threshold circuits and the lamp motor and heater are disconnected from the power supply the ventilator remains connected, if the voltage across the temperature varying resistance is outside of the two limits set by the threshold circuits.

O v United States Patent [1 51 3,705,289 Szostak et al; 1 Dec. 5, 1972 [54] ELECTRONIC COPYING EQUIPMENT 1 WITH CONTROLLED HEATER [56] References Cited [72] Inventors: Roland Szostak, Grunwald; Karl UNITED STATES PATENTS Hartwig, Unterhaching; Gunter Maurischat, Munich, all of Germany 3,505,497 4/1970 Lawes et al ..2l9/2l6 [73] Assignee: Agfa-Gevaert Aktiengesellschaft, Primary Examiner c Albritton Leverkusen Germany Attorney-Michael S. Striker [22] Filed: Sept. 23, 1971 21 Appl. No.: 182,986 [57] ABSTRACT A temperature varying resistor is in one branch of a bridge circuit. The voltage across the diagonal of the 30 l 1 Forelgn Application Emmy Data bridge controls the .heater cut in and cut off under Sept. 30, 1970 Germany ..P 20 48 076.6 normal operation, The voltage across the temperature I varying resistor is applied to two threshold circuits and US. C.- .Q 19, the lamp motor and heater are disconnected from the 250/65 263/6 E power supply the ventilator remains connected, if the [5 1] Int. Cl. ..H05b 3/00 voltage across the temperature varying resistance is [58] Field of Search ..2l9/216, 481 -482,

219/489-490, 49.4, 519; 250/65 T, 65 ZE; 263/6 E; 118/637,- 641, 642, 620; l17/l7.5

outside of the two limits set by the threshold circuits.

8 Claims,'l Drawing Figure ELECTRONIC COPYING EQUIPMENT WITI-I CONTROLLED HEATER BACKGROUND OF THE INVENTION a predescribed value. The difficulty with this type of arrangement is that if for example a short circuit should occur in this relatively sensitive unit, the internal resistance of the temperature varying resistor would be sharply decreased or even become zero, thus indicating, incorrectly, that a low operating temperature exists. The heater is thus disconnected from the energizing source either too late or not at all. The equipment is thus heatedabove safe limits which causes either a yellowing or a blacking of the copy or, if the error is not notedin time might even result in destruction of the heating coil. Of course, on the other hand, if there is a break within the temperature varying resistance or in its terminal connections, the apparent internal resistance of the temperature varying resistor is increased and may reach an infinite value, thereby indicating, again incorrectly, an excessively high temperature. The heater is thus controlled in such a manner that an excessively low operating temperature results, causing the reproductions to be insufficiently fixed.

SUMMARY OF THE INVENTION It is an object of the present invention to furnish a heater'control circuit which will prevent the abovementioned difficulties.

:It is a further object of this invention to accomplish the above-mentioned objective with a minimum of equipment.

The present invention is a heater control circuit in electrostatic copying equipment which has heating means for providing the heat required for the fusing of the powder image on the transfer material. It comprises bridge circuit means having a first and second branch. A first and second bridge output terminal are located, respectively, in said first and second branch, said first branch further comprising means varying the signal at said first bridge output terminal as a function of temperature. Energizing means are provided for energizing.

said heating means. First control means are connected to said bridge circuit means and connect and disconnect said heating means from said energizing means in dependence upon the difference and signal at said first and second bridge output terminal. Finally, second control means are connected to said first bridge output terminal for disconnecting said heating means from said energizing means when said signal at said first bridge output terminal is outside of a predetermined signal range.

In the above-described arrangement the heating means are disconnected from the energizing means whenever the resistance of the temperature varying resistance means is outside of the limits determined by the range of possible operating temperature of the equipment, that is, substantially between room temperature and a temperature lying somewhat above the highest permissible operating temperature. The remainder of the copying apparatus, namely the lamp and the motor driving the drum carrying the transfer material is also deenergized. However, the ventilator remains energized even after the first and second control means have operated. Thus all malfunctions in the temperature varying resistor are noted and the equipment is disconnected before any damage to either the copies being generated or the equipment-itself can result. i

In a preferred embodiment of the present invention the first and second control means comprise a relay whose contacts disconnect and connect the equipment from the energizing source upon receipt of corresponding operating signals. This relay may be energized or deenergized (furnished operating signals) as a function of the state of a flip-flop (bistable means). The flip-flop may, for example, be set by the output signal of two threshold circuits. Thereby the equipment will remain disconnected until the operator presses a switch provided for resetting of the flip-flop.

As mentioned above, the first control means; which function under normal conditions within the temperature varying resistor connect and disconnect only the heating coil from the energizing source. The second control means, which operate when a fault exists in the temperature varying resistance means disconnect also the motor driving the drum and the lamp. The ventilator remains energized under all conditions so that any possible overheating of either the heating coil, the drum or-the transfer material will be removed as soon as possible.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when'read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE shows a preferred embodiment of the heater control arrangement of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described with reference to the drawing.

As shown in the FIGURE, the bridge circuit means have a first branch comprising a resistance R, connected in series with the temperature varying resistance means, here a resistance labelled P,. The first bridge circuit output terminal is labelled 25 and is the common point of resistor R, and P The second branch of the bridge circuit means comprises a resistance R, series connected to a variable resistance R;,. The common point of resistors R and R constitutes the second bridge output terminal and is connected to a line labelled 1 in the FIGURE. In the example shown in the FIGURE the bridge is energized by a 24 volt source, one side of which is at ground potential.

The first control means comprise an amplifier 3 which may for example be a differential amplifier and which has an input connected to line 1 and a second input connected to terminal 25. The output of amplifier 3 energizes first relay means, 4, whose contacts 5 serve to connect 1 and, 6, of heating coil 23 to energizing means, here an AC voltage of 220 volts. The other side, 7, of the heating coil is directly connected to the 220 volt source.

- The first bridge output terminal is further connected via line 2, 7 and 8 to the input of first threshold means, 10, and the lines 2, 7 and 9 to the input of second threshold means, 11. The reference voltages to the threshold means are respectively provided by a variable DC voltage U and a variable DC voltage U respectively. The output of the first threshold means is furnished on a line 13, that of the second threshold means on a line 14. Lines 13 and 14 are connected in common to a line 12 which is in turn connected to-the input of a bistable stage 30. The output of the bistable stage serves to control a relay 15 (second relay means). Relay 15 in turn controls a switch 16 which, when closed connects motor 19, lamp 20 and heating coil 23 to the 22 0 volt source and when open disconnects thesefromsaid source.

If .the bistable circuit 30 is set by'the signal on the above-described line 12, then the reset input is activated by the depression of a key 21 which, when closed, connects said reset to a line 22 which in turn is connected to the 24 volt side of the battery energizing the bridge circuit.

The ventilator or fan 24 is directly connected to the 220 volt source without passing through the contacts of relay 15. This ventilator thus remains energized even after the remainder of the equipment is disconnected. The temperature of the equipment, if it has excessively increased, is thus brought to safe values as soon as possible.

The above-described equipment operates as follows: The bridge circuit is adjusted via resistance 3 so that no voltage exists between the first and second bridge output terminals (lines 1, 2) when the resistance of the temperature varying resistor P, corresponds to. the desired operating temperature of heating coil 23. When the resistance of resistor P, decreases because of a drop in temperature under a predetermined temperature, amplifier 3 energizes relay 4 causing contacts 5 to be closed. The heater coil is now energized and continues to operate until the upper temperature limit of the desired operating temperature is reached and the resistance of resistor P, exceeds the value for which the bridge was balanced. If a malfunction develops in resistor P,, its resistance value and therefore the voltage across it exceeds the voltage appearing across it under normal operating conditions. For example the voltage across the resistor will decrease to substantially zero in case of a short-circuit, thereby being less than the voltage U, constituting the reference voltage for the first threshold circuit 10. A signal will then appear on line 13 which will set flip-flop 30, operating relay 15 to open contact 16. In case of a break in resistor P, the voltage at the first bridge output terminal will increase beyond the second reference voltage U; thereby causing a signal to appear on line 14 at the output of the second threshold means. The signal at the output of the second threshold means will operate to open contacts 16 in exactly the same manner as the signal appearing at the output of the first threshold means. Thus a-signal at the output of either the first or second threshold means will cause the lamp, motor and heater coil to be de-energized while the ventilator 24 continues to operate. When the fault in the temperature varying resistor has been removed, or the resistor replaced, the operator pushes key 21 causing flip-flop to be reset, operating relay 15 to close contact 16 and the normal operation of the equipment continues.

While the invention has been illustrated and described as embodied in circuitry using particular first and second control means, it is not intended to be limited to the details shown, since various modifications, structural changes and circuit changes may be made without departing in any way from the spirit of the present invention.

' Without further analysis the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of 'prior art, fairly constitute essential characteristicsof the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

We claim:

1. In electrostatic copying means having transfer material, a powder image created on said transfer material, and heating means for fusing said powder image, a control arrangement for said heating means comprising, in combination, bridge circuit means having a first and second branch and a first and second bridge output terminal in said first and second branch respectively, said first branch comprising means varying the signal at said first bridge output terminal as a function of temperature; energizing means for energizing said heating means; first control means connected between said first and second bridge output terminal for connecting said heating means to said energizing means and disconnecting said heating means from said energizing means in dependence upon the difference in signal at said first and second bridge output terminal; and second control means connected to said first bridge output terminal for disconnecting said heating means from said energizing means when said signal at said first bridge output terminal is outside of a predetermined signal range.

2. Electrostatic copying means as set forth in claim 1, wherein said means varying said signal at said first bridge output terminal as a function of temperature comprise temperature varying resistance means.

3. Electrostatic copying means as set forth in claim 2, wherein said first control means comprise amplifier means, and first relay means having a coil connected to the output of said amplifier means and first relay contact means interconnected between said heating means and said energizing means.

4. Electrostatic copying means as set forth in claim 3, wherein said amplifier means comprise differential amplifier means.

5. Electrostatic copying means as set forth in claim 1, wherein said second control means comprise first and second threshold means, each having an input connected to said first bridge output terminal, said first and second threshold means respectively furnishing a first and second threshold output signal in response to a signal at said first bridge output terminal respectively exceeding a predetermined maximum signal and less than a predetermined minimum signal.

6. Electrostatic copying means as set forth in claim 5, wherein said second control means further comprise bistable means connected to said first and second threshold means for furnish a bistable output signal in response to' said first or second output signal; and second relay means disconnecting said heating means 

1. In electrostatic copying means having transfer material, a powder image created on said transfer material, and heating means for fusing said powder image, a control arrangement for said heating means comprising, in combination, bridge circuit means having a first and second branch and a first and second bridge output terminal in said first and second branch respectively, said first branch comprising means varying the signal at said first bridge output terminal as a function of temperature; energizing means for energizing said heating means; first control means connected between said first and second bridge output terminal for connecting said heating means to said energizing means and disconnecting said heating means from said energizing means in dependence upon the difference in signal at said first and second bridge output terminal; and second control means connected to said first bridge output terminal for disconnecting said heating means from said energizing means when said signal at said first bridge output terminal is outside of a predetermined signal range.
 2. Electrostatic copying means as set forth in claim 1, wherein said means varying said signal at said first bridge output terminal as a function of temperature comprise temperature varying resistance means.
 3. Electrostatic copying means as set forth in claim 2, wherein said first control means comprise amplifier means, and first relay means having a coil connected to the output of said amplifier means and first relay contact means interconnected between said heating means and said energizing means.
 4. Electrostatic copying means as set forth in claim 3, wherein said amplifier means comprise differential amplifier means.
 5. Electrostatic copying means as set forth in claim 1, wherein said second control means comprise first and second threshold means, each having an input connected to said first bridge output terminal, said first and second threshold means respectively furnishing a first and second threshold output signal in response to a signal at said first bridge output terminal respectively exceeding a predetermined maximum signal and less than a predetermined minimum signal.
 6. Electrostatic copying means as set forth in claim 5, wherein said second control means further comprise bistable means connected to said first and second threshold means for furnish a bistable output signal in response to said first or second output signal; and second relay means disconnecting said heating means from said energizing means in response to said bistable output signal.
 7. Electrostatic copying means as set forth in claim 6, wherein said electrostatic copying means further comprise motor means and, lamp means and ventilator means; means connecting said ventilator means directly to said energizing means; and wherein said second relay means comprises relay contacts disconnecting said lamp means and said heating means in response to said bistable output signal.
 8. Electrostatic copying means as set forth in claim 7, further comprising means resetting said bistable means following the furnishing of said bistable output signal. 